The present invention refers to a venting valve assembly for casting moulds, comprising a venting valve having a valve housing, a venting channel located in the interior of the valve housing, and a valve closure member located in the interior of the valve housing and adapted to move between an open position and a closed position.
In order to reliably avoid the occurrence of air inclusions in the finished casting during the casting operation, the mould and the cavity in the mould, respectively, have to be vented during the casting operation. Thereby, not only the air contained in the cavity of the mould has to be allowed to escape, but in addition it must be ensured that also the gases escaping from the liquid casting material are removed from the mould cavity.
One of the problems in connection with venting die casting moulds can be seen in the requirement that the venting valve of the valve assembly be closed as late as possible in order to ensure that the mould cavity is vented until it is fully filled with liquid casting material, but that it is also to be avoided that liquid casting material enters the venting valve.
In order to take this problem into account, generally two kinds of valve assemblies for die casting moulds are known, whereby in either case a venting valve is provided that is equipped with a axially back and forth movable valve piston for closing the venting channel. While the valve piston is moved by suitable driving means in a first kind of valve assemblies, the valve piston of a second kind of valve assemblies is operatively connected to a power pick-up member that is operated directly by the liquid casting material flowing from the cavity of the mould into the venting channel, thereby making use of its inherent kinetic energy.
Suitable driving means for the above mentioned first kind of valve assemblies may include pneumatically or hydraulically operated driving systems for moving the valve piston. The moment in which the closing of the venting valve is initiated can be determined, for example, by means of a sensor that monitors the level of the mould cavity. However, one difficulty observed with such systems consists in the fact that the closing operation takes a considerably long time because the signal initiating the closing operation, mostly an electric signal, has to be transformed into a mechanical movement, for example into the operation of a servo valve. Moreover, for the purpose of closing the venting valve or for the purpose of operating an actuating member that is operatively connected to the valve piston of the venting valve, a predetermined system pressure must be available in order to ensure that the venting valve can be pneumatically or hydraulically closed within the required time period. However, since the operation of a servo valve usually causes a drop in system pressure, it is necessary to rebuild the system pressure again before the servo valve can be closed. Finally, in most cases, a locking mechanism holding the valve piston in its open position has to be operated, resulting in an additional delay of the closing operation. It is understood that such valve assemblies are of a quite complicated design and require a high expenditure; moreover, they are subject to be influenced by certain operation parameters. Moreover, such valve assemblies usually require at least appr. 10 milliseconds from the detection of the intruding casting material to reach the fully closed position of the valve assembly.
In contrary, with the second kind of valve assemblies, it is possible to realize very quick acting and reliable venting devices. In order to ensure that a ram pressure can be built up that is high enough to operate the venting valve piston, the venting channel leading from the mould cavity to the power pick-up member is provided with a number of deviations and constrictions. Moreover, the venting channel must have a certain minimal distance and has to be of angled design between the power pick-up member and the real valve body member of the venting valve, in order to ensure that the venting valve is safely closed before the liquid casting material has reached the venting valve. In order to increase the efficiency of such valve assemblies, usually a vacuum pump is connected to the venting valve.
The document EP 0 612 573 discloses a valve assembly referred to herein for venting diecasting moulds, comprising a venting channel, a venting valve located in the venting channel and an operating means for closing the venting valve. The operating means comprises an impact transmitter that is exposed to the liquid casting material advancing from the mould cavity into the venting channel. The impact transmitter is mechanically operationally coupled to the movable closure element of the venting valve. Thereby, the impact transmitter is designed as a push member having an operating stroke that is limited to a fraction of the stroke to be passed through by the movable element of the venting valve. Moreover, the closure element of the venting valve is freely movable along the path exceeding the operating stroke of the impact transmitter, and the operating means comprises a power transmission member for transmitting the impact impulse from the impact transmitter to the movable closure member of the venting valve.
Even if such a venting valve assembly operates very reliably in practice, it remains desirable in certain applications that the energy required for closing the venting valve would not be supplied from the moving casting material alone. As is clearly evident from the fundamental formula for calculating the kinetic energy (E=mxc2x7v2/2), the energy available for closing the venting valve depends on the mass and on the velocity of the casting material. In other words, that means that the available energy might not be sufficient under certain unfavorable operating conditions, particularly in the case of low casting material mass and/or low flow velocity of the fluid casting material to close the venting valve within the required time period. On the other side, in the case of high casting masses and/or high flowing velocities thereof, a high energy impact can act on the impact transmitter, with the result that the transmitter and the closure member hit the end stop and/or the valve seat with a high speed; in view of good reliability and long service life of the venting valve assembly, this is highly undesirable.
Therefore, it is an object of the present invention to provide a venting valve assembly for casting moulds which can be universally used due to the fact that its closure element is moved very quickly from the open position to the closed position, independent from the casting operation parameters, i.e. independent of the design of the casting apparatus and/or the nature of the casting material.
To meet this and other objects, the present invention provides a venting valve assembly for casting moulds, comprising a venting valve having a valve housing and a venting chamber located in the valve housing, a venting channel located in the interior of the valve housing and communicating with the venting chamber, and a valve closure member located in the interior of the valve housing and adapted to move between an open position in which the venting channel connects the venting chamber with the ambient atmosphere, and a closed position in which the venting channel seals the venting chamber against the ambient atmosphere.
Moreover, the venting valve assembly comprises first means for frictionally locking the valve closure member in the open position and second means for biasing the valve closure member towards the closed position when the valve closure member is in its frictionally locked open position.
With such a venting valve assembly, the valve closure member can be brought into its closed position extremely quickly, because the time required for releasing the frictional lock is very short and because the valve closure member is already biased to quickly move in its closed position.
In a preferred embodiment of the venting valve assembly, a venting valve is provided having a chamber that can be hydraulically or pneumatically pressurized and that incorporates a wall portion bulging towards the closure member in response to the overpressure in the chamber to thereby frictionally lock the closure member in its open position. Thereby, the wall portion is elastically deformable to such a degree within the limits of elasticity of the material it is made of that it returns to its former undeformed shape once the hydraulic or pneumatic overpressure in the chamber is reduced, thereby releasing the valve closure member. Compared to venting valves disclosed in the prior art, it is not required to build-up a pressure and/or to bring a closure member from a locked into an unlocked state first in order to enable the closure member to move from its open to its closed position; rather, only the pressure in the chamber has to be reduced to such a degree that the wall portion of the chamber moves elastically back to release the closure member which, in turn, suddenly moves from its open to its closed position. Such pressure reduction can be performed, for instance, by actuating a release valve. The entire closing time of the valve assembly, counted from the detection of the casting material in the venting channel up to the completion of the closing of the valve, can be substantially reduced, as compared to previously known venting valve assemblies.